Thermal circuit interrupter



1946- E. M. THIELERS ET AL 2,410,370 THERMAL CIRCUIT INTERRUPTER Filed Sept. 20, 1943 L I O J K a M 1m A N Q Fig.2 Fig.3

' XNVE TQRs m, E..M TH\ELERS F 4 P- WO-W. BROBERG I9. 'Bq

- sulating the spindle from the cover A,

Patented Oct. 29, 1946 THERMAL CIRCUIT INTERRUPTER Ernst Martin Thielers, Stockholm, and Walter Otto Wilhelm Broberg,

Nynashamn, Sweden,

assignors to Telefonaktiebolaget L. M. Ericsson,

Stockholm, Sweden, a co Application September 20, 1943,

mpany of Sweden Serial No. 503,118

In Sweden October 20, 1942 Within telephone technic a fuse heat-coil has been used comprising a heat-producing winding acting upon an interruption point which consists of two metal parts, soldered together by an easilyfusible alloy. As a rule,.the heat-producing coil is inserted inv ametalcover and wound on a narrow metal spindle bored lengthways, from one side so as to receive a pin which, by a fusible alloy, is soldered to the wall of the boring (interruption poin i When current fiowsthrough the heat-producing winding, heating of the easily-fusible solder follows which, at a pre-determined current, causes the solder to soften or tO-fllse-So that the pin, actuated by an elastic force, can be pulled out of the boring thereby causing interruption of the current. At the same time, the spring adapted to pull out the'pin may be brought against a contact and close an alarm circuit.

An arrangement of this kind is shown in Figs.'1 and 2 on the drawing, Fig. 1 showing the heatcoil and Fig. 2 the heat-coil mounted in springs on a fitting.

Fig. 3 is a diagram of a circuit with the heatgoils combined with lightning protectors and Fig. 4 is a diagram representing the characteristic curve of the heat-coils. 1

Fig. 5 is a sectional view of an embodiment of the invention.

In Fig. 1 which shows a heat-coil in, cross-section view, A isa metal cover, B an insulating sleeve in the shape of a truncated cone, generally made of ebonite or-fiber, the middle of which is threaded so that the spindle can be screwed in to the insulating sleeve; E is insulating washers, in-

F a .pin with a, head; w-hichpin is inserted in C and fixed by an easily-fusible solder (fusing point being normally about 70 0.). Gjis a heat-producing winding whose one end is spindle C and whose other end is soldered to the coveratI-I. m

The outer terminal clamp of the heat-coil is formed partly by the cover A, partly by the pin F.

As a rule, the heat-coils are mounted in socalled strips and fastened by means of springs, as

in contact with the 8 Claims. (Cl. 200123) shown in Fig. 2. In this figure, J and K are two springs, K being provided with a slot in which the narrow part of the heat-coil cover A may be inserted. This causes the springs J and K to be pushed apart and good contactis thus obtained. L is a spring which'in its normal rel-axed position rests against a strip M and at its upper end has a slot in which the pin F is to be inserted. The contact at the strip M is then broken and the spring L presses against the pin head, this pressure generally amounting from 250 to 300 g.

The described heat-coil operates as follows. The soldering metal which fixes the pin F to the spindle C softens at a pre-determined rise of temperature, caused by excess current, whereupon the spring L is released thereby pulling out the pin. Current interruption follows and spring L is applied against the strip M, thus closing an alarm circuit.

The heat-coils are most frequently used in telephone exchanges where they are placed in connection with main distributing frames. The

' heat-coils are frequently combined with lightning protectors and fuses according to Fig. 3, where the incoming line is marked N, the fuses O, lightning protectors A, heat-coils P and the line to internal exchange devices Q. In this combination the fuse generally has a limiting current of 3 9... Whilst the heat-coil is adapted to interrupt at lower values.

Due to its structure, the heat-coil has characteristic curves differing from the characteristic curves of a fuse consisting of a fuse-wire. Fig. 4 gives the characteristic curves of one of the most frequently used kinds of heat-coils. It has a limiting current of about 0.16 a. and, at a. load of 0.25 a., an operating time of about 20 seconds and at 0.7 a. an operating time of 2 seconds. The heat-coil is thus considerably slower than a fuse containing a fuse-wire. The resistance of that coil is about 20 ohms.

Owing to said characteristic curves the heatcoil is adapted for use in combination with a fuse (fuse-wire) of about 3 a., the fuse-wire operating at high excess currents which generally are of short duration, while the heat-coil is adapted to interrupt at lower non-allowable currents which sometimes may be of longer duration.

The heat-coil thus described has however several drawbacks. Primarily, the resistance is high Fusing current, ampcres at a fusing time of 20 seconds Resistance Still more inconvenient is however the following observation made by the inventors. It has been found that the heat-coil has fairly narrow limits which with the heat-coil having a resistance of 20 ohms, amounts to about 0.16 and about 1 a. When the current exceeds 1 a. it may occur that (l) The insulation of the winding wire carbonizes and short-circuits occur without the pin being pulled out and causing alarm. The heat-coil then offers no protection.

(2) The winding wire is burnt Without the pin being pulled out and giving alarm.

When these heat-coils are used in combination with tube fuses of 3 a. capacities a field of the 3 intensity of current of between 1-3 a. exists within which no protectors, operating reliably, are available.

After a close research the inventors have succeeded in constructing a heat-coil, whose resistance amounts to less than half of that of the above described heat-coil type and which for all intensities of current above the limiting intensity break the circuit and causes alarm and which may be inserted in the same strips as those used for the present heat-coils. The invention is characterized in that, by excess current, the point of interruption (i. e. the solder) attains the fusingor softening temperature earlier or quite as early as the heat-producing winding attain a temperature injurious to the wire or its insulation. This is obtained by reducing the heat capacity as much as possible and by counteracting the heat-conduction from the pin to the bottom of the cover. Heat losses caused by radiation are left out of consideration by this invention.

Fig. 5 shows an embodiment of the invention. R is a cover of metal, S a thin tube of metal (preferably with good heat conductivity). T indicates insulation washers, for example of fiber, which are secured at the ends of the tube, the tube ends for this purpose being stamped to form a small flange. U is the pin arranged in the tube and being soldered with easily-fusible alloy. Finally, V is the heat-producing winding, whose ends are soldered to the tube or the cover, respectively. The parts maintain their position by means of several annular depressions X formed in the cover.

The winding is arranged so as to be wound in as few layers as possible (preferably one layer) and as close to the tube as possible. It is also desirable to coat the winding with some fixing agent (for instance thin cellone solution). The heat capacity of the winding must not be too high. Consequently, the wire diameter should be selected rather small as, for instance, with a coil having a resistance of 10 ohms it may be about 0.10 mm.

It should also be observed that, at the points where the heat-coil rests against the springs, the surfaces of contact should be made as small as possible. For that purpose the pin head in its inner portion has a conical form. Further, the circular slot at the lower part of the cover may be conically shaped. When the heat-coil is fastened in its springs the above described springs I and K will rest against small surfaces on both sides of the lower flange of the cover.

Easily-fusible alloys with various fusing temperatures may serve as soldering metal. Normally, an alloy with a fusing point of 65-70" C. is used.

The described heat-coil is adapted for use preferably in telephone technics but may be used also in other fields, where its working properties may be found useful.

We claim:

1. In a device of the character described, an electrically conductive cover, an electrically eonductive tube arranged within the cover, insulating means supporting said tube in said cover, a pin soldered in said tube, a heating coil wound on said tube for fusing the pin solder, the pin and cover being engageable with fastening springs and the heat capacity of the heating coil and parts heated by said coil and heat losses of said insulating means and fastening springs being so reduced that the interruption point for all excess currents over the limiting current attains its fusing temperature at least as early as the leading coil attains a temperature injurious thereto.

2. In a device of the character described, an electrically conductive cover, an electrically conductive tube arranged within the cover, relatively thin disc like insulating members supporting the tube within the cover, a .pinsoldered in said tube and a heating coil wound on said tube for fusing the pin solder.

3. An arrangement as claimed-in claim ,2 wherein the thickness of the wall of said tube does not exceed .5 mm.

4. An arrangement as claimed in claim 2 wherein the internal diameter of said tube is approximately 1 mm.

5. An arrangement as claimed in claim 2 wherein the heating coil is coated with a thin fixing agent.

6. An arrangement as claimed in claim 2 wherein the ends of the tube are flanged and engaged with the disc like insulating members.

'7. An arrangement as claimed in claim 2 wherein the ends of the tube are flanged and engaged with the disc like insulating members, said cover having depressions therein for retaining the insulating members in position.

8. An arrangement as claimed in claim 2 wherein the pin and cover are provided with portions engageable with fastening springs, said portions being shaped to minimize the area of contact with said springs.

ERNST MARTIN THIELERS. WALTER OTTO WILIIEL'M BROBERG. 

